1. Field of the Invention
The invention relates to an apparatus for applying a suspension containing oxide particles dispersed in a liquid to a plate for magnet sintering, and a method for manufacturing a rare earth magnet using such an application apparatus. More particularly, the present invention relates to an apparatus for applying a homogenized suspension to a plate, and a method for manufacturing a rare earth magnet using such an application apparatus.
2. Description of Related Art
A rare earth sintered magnet is manufactured by pulverizing an alloy for a rare earth magnet (material alloy) to produce alloy powder and compacting the alloy powder, followed by a sintering process and an aging heating process. Presently, two types of rare earth magnets, samarium-cobalt magnets and neodymium-iron-boron magnets, are widely used in various fields. In particular, neodymium-iron-boron magnets (hereinafter, referred to as Rxe2x80x94Txe2x80x94(M)xe2x80x94B magnets where R denotes a rare earth element and/or yttrium (Y), T denotes a transition metal selected from the group consisting of iron (Fe), cobalt (Co), and nickel (Ni), M denotes an additive element, and B denotes boron or a compound of boron and carbon) have found active applications to various types of electronic equipment because the Rxe2x80x94Txe2x80x94(M)xe2x80x94B magnets exhibit the highest maximum magnetic energy product among various other types of magnets and are comparatively inexpensive.
During the sintering process of magnet manufacturing, green compacts are mounted on a sintering plate made of a highly heat-resistant material such as stainless steel and molybdenum. The sintering plate is then placed in a sintering furnace where the green compacts are heated to a high temperature (for example, 1000 to 1100xc2x0 C.) in an inert gas atmosphere. The heated compacts are sintered and shrunk to form a rare earth sintered magnet.
During the sintering process, if a green compact is directly mounted on the sintering plate, the green compact and the plate may be locally welded together. This is because a rare earth element such as Nd is used as a constituent of the Rxe2x80x94Txe2x80x94(M)xe2x80x94B magnet and causes a eutectic reaction with a metal element contained in the plate at a temperature less than a sintering temperature. Once local welding occurs between the plate and the compact, the compact fails to shrink smoothly during the sintering, resulting in the generation of cracks and chips in the sintered body. Even if such welding between the compact and the plate does not occur, the compact may crack on the surface due to friction between the plate and the compact (sintered body). Moreover, a product from the eutectic reaction may attach to the sintering plate. In such a case, it takes time and effort to remove the attachment from the plate when the plate is reused.
In order to prevent the welding between the sintering plate and the green compact, there is conventionally known a sintering method where powder is spread over the sintering plate and green compacts are mounted on the powder spread on the sintering plate (For example, Japanese Laid-Open Patent Publication No. 4-154903). The spreading powder used is made of a material that has a low reactivity with the green compact and a good stability at high temperatures. For example, when the green compact contains a rare earth metal, the spreading powder is made of a material that has a low reactivity with the rare earth metal, such as a rare earth oxide (for example, neodymium oxide). By using such a spreading powder, it is possible to prevent welding between the plate and the green compact, and thus prevent occurrence of breakage such as cracking and deformation on the surface of the resultant rare earth magnet.
There are known methods for spreading powder on the plate, including a method where the powder is sprayed onto the plate using LP gas, a method where the powder is dispersed in a volatile dispersion medium such as ethanol and the resultant dispersion medium (i.e., suspension) is applied to the plate, and a method described in Japanese Laid-Open Patent Publication No. 11-54353, where an organic solvent such as ethanol and acetone is added to the powder made of Dy2O3 or CaF2 to form a slurry, and the slurry is applied to the plate with a brush and the like.
The above conventional methods have the following problems. The method using gas to spray powder finds difficulty in spreading the powder uniformly on the plate. If the powder is not spread uniformly on the plate, a green compact may partly be welded with the plate during the sintering, and friction (resistance) between the compact and the plate occurring during shrinkage of the compact may vary with the position. These result in the compact failing to shrink uniformly. As a result, breakage (cracking and the like) and undesirable deformation are generated in the compact. In particular, when elongated, the compact fails to shrink uniformly and thus cracking and deformation are easily generated.
In the method where a suspension containing powder particles in a volatile liquid such as ethanol, or a slurry of powder with an organic solvent added thereto, is applied to the plate with a brush and the like, the work of applying the suspension or the slurry to the plate is time-consuming, and thus the productivity is low. In addition, in order to spread powder uniformly on the plate, the suspension or the slurry must be applied to the plate in the form of a thin layer. Applying such a suspension or slurry uniformly to the plate is difficult.
In the case of dispersing a powder of a rare earth oxide and the like in a volatile liquid such as ethanol, the powder is easily separated from the liquid in the suspension because the difference in specific gravity between the volatile liquid and the powder particles is comparatively large (for example, the specific gravity of ethanol is 0.8 while that of R2O3 (rare earth oxide) is 7 to 8). Using such a suspension, it is difficult to maintain a uniform concentration of the powder particles in the entire suspension. Therefore, even if the suspension is successfully applied uniformly to the plate, the concentration of the applied suspension often varies with position. It is therefore difficult to spread powder particles uniformly on the plate by applying such a suspension. If uniform spreading of powder fails, the resultant sintered body tends to have breakage and undesirable deformation.
Moreover, in the case of automatically applying a suspension or a slurry to the plate from a tank via a pipe or the like, the pipe may possibly become clogged. In particular, for intermittent application with stops interposed between plates, the supply of the suspension or the slurry is temporarily stopped or delayed. This causes poor flowability of the suspension or the slurry in the pipe, and thus the powder particles in the suspension or the slurry tend to settle, resulting in clogging of the pipe.
A main object of the present invention is to provide an application apparatus capable of applying a suspension containing powder particles (spreading powder particles) of an oxide dispersed in a liquid to a sintering plate uniformly without clogging a transport path such as a pipe and a tube with the suspension, to enable uniform spreading of the oxide powder on the plate.
Another object of the present invention is to provide a method for manufacturing a rare earth magnet where oxide particles are spread uniformly on a sintering plate using the application apparatus described above so that cracks or the like are not generated in the green compacts mounted on the plate during sintering.
The suspension application apparatus of the present invention is an apparatus for applying a suspension containing powder particles of an oxide dispersed in a liquid to a plate for magnet sintering, where the powder particles have a specific gravity greater than the liquid. The apparatus includes a container for storing the suspension; a stirrer for stirring the suspension stored in the container, a transport path through which the suspension is transported from the container to the plate, and a homogenizer for homogenizing the suspension by applying a mechanical force to at least part of the suspension flowing through the transport path.
In a preferred embodiment, the homogenizer generates unsteady flow in the at least part of the suspension flowing through the transport path.
In a preferred embodiment, the unsteady flow is a flow in the direction opposite to the direction from the container toward the plate.
In a preferred embodiment, the suspension application apparatus further includes a discharge path connected to the transport path for enabling discharge of the suspension flowing in the opposite direction.
In a preferred embodiment, the homogenizer can jet a fluid into the suspension flowing through the transport path.
In a preferred embodiment, the fluid is air.
In a preferred embodiment, the suspension application apparatus further includes a discharge path connected to the transport path for enabling discharge of at least part of the fluid.
In a preferred embodiment, the discharge path extends as far as the inside of the container.
In a preferred embodiment, the homogenizer can generate unsteady flow in at least part of the suspension in the vicinity of a connection between the transport path and the container.
In a preferred embodiment, the suspension application further includes a metering pump provided at a position of the transport path downstream of the homogenizer.
In a preferred embodiment, the suspension application apparatus further includes a spreading device for spreading the suspension supplied to the surface of the plate over the surface.
In a preferred embodiment, the spreading device includes an absorptive roller provided to come in contact with the surface of the plate.
In a preferred embodiment, the homogenizer applies a mechanical force to the transport path.
In a preferred embodiment, the homogenizer swings the transport path.
In a preferred embodiment, the suspension application apparatus further includes a plate cleaner for cleaning the plate prior to the application of the suspension, wherein the plate cleaner includes a powder shooter for allowing powder to impinge against the plate and a swinger for swinging the powder shooter, and the homogenizer is connected with the swinger of the plate cleaner, so that the transport path is swung with the movement of the swinger.
In a preferred embodiment, the suspension application apparatus further includes a nozzle connected to an end of the transport path, a gas supply path connected to the nozzle, wherein the suspension is sprayed onto the plate using a gas supplied to the nozzle through the gas supply path.
In a preferred embodiment, the liquid is volatile.
In a preferred embodiment, the powder particles of an oxide comprises powder particles of a rare earth oxide.
The method for manufacturing a rare earth magnet of the present invention includes the steps of preparing a plate for magnet sintering, applying a suspension containing powder particles of an oxide in a liquid to the plate using any of the suspension application apparatus described above, mounting a green compact produced by compacting alloy powder for a rare earth magnet on the plate to which the suspension has been applied, and sintering the green compact mounted on the plate.
In a preferred embodiment, the surface roughness Rmax of the plate is in a range of 1 xcexcm to 300 xcexcm.
In a preferred embodiment, the surface roughness Ra of the plate is in a range of 0.1 xcexcm to 150 xcexcm.
In a preferred embodiment, the concentration of the suspension is in a range of 200 g/L to 500 g/L.
The method for manufacturing a rare earth magnet of the present invention includes the steps of: preparing a plate for magnet sintering; applying a suspension containing powder particles of an oxide dispersed in a liquid to the plate, the powder particles having a specific gravity greater than the liquid; mounting a green compact produced by compacting alloy powder for a rare earth magnet on the plate to which the suspension has been applied; and sintering the green compact mounted on the plate. The surface roughness Rmax of the plate is in a range of 1 xcexcm to 300 xcexcm.
The method for manufacturing a rare earth magnet of the present invention includes the steps of: preparing a plate for magnet sintering; applying a suspension containing powder particles of an oxide dispersed in a liquid to the plate, the powder particles having a specific gravity greater than the liquid; mounting a green compact produced by compacting alloy powder for a rare earth magnet on the plate to which the suspension has been applied; and sintering the green compact mounted on the plate. The surface roughness Ra of the plate is in a range of 0.1 xcexcm to 150 xcexcm.
In a preferred embodiment, the concentration of the suspension is in a range of 200 g/L to 500 g/L.
In a preferred embodiment, the average particle size of the powder particles is in a range of 1 xcexcm to 20 xcexcm.
As used herein, the term xe2x80x9csuspensionxe2x80x9d refers to a suspension obtained by dispersing powder in a liquid, including the state where powder particles is scattered in the liquid in a nonuniform manner and the state where part of the powder particles is settled out.